Transparent Clocking in a Cross Connect System

ABSTRACT

A cross connect apparatus or system with transparent clocking, consistent with embodiments described herein, connects a selected source or ingress port to a selected destination or egress port and clocks data out of the selected egress port using a synthesized clock that is adjusted to match a recovered clock from the selected ingress port. A transparent clocking system may generate the synthesized clock signal with adjustments in response to a parts per million (PPM) rate detected for the associated recovered clock signal provided by the selected ingress port. The cross connect system with transparent clocking may be a 400G cross connect system with 10G resolution. The cross connect system with transparent clocking may be used in optical transport network (OTN) applications, for example, to provide an aggregator and/or an add-drop multiplexer (ADM) or to provide a reconfigurable optical add-drop multiplexer (ROADM) upgrade to a higher data rate.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15,474,561, filed Mar. 30, 2017, now U.S. Pub. No. 2017-0288849, whichclaims the benefit of U.S. Provisional Application Ser. No. 62/317,194filed on Apr. 1, 2016, which is fully incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present disclosure relates to cross connect systems and moreparticularly, to transparent clocking in a cross connect system.

BACKGROUND INFORMATION

A cross connect system may be used to connect any one of a plurality ofsource or ingress ports to any one of a plurality of destination oregress ports. Data received from a source device coupled to the selectedsource/ingress port may thus be connected to the selecteddestination/egress port for transmission to a destination device.Optical cross connects, for example, may be used to reconfigure opticalnetworks dynamically, for example, to manage traffic on the networks.Electrical-switching-based optical cross connects convert optical datasignals to electrical data signals, perform electrical switching of thedata signals between the ports, and then convert the electrical datasignals back to optical data signals.

When electrical data signals are received, clock signals are recoveredfrom the data signals and the recovered clock signals are used to clockthe recovered data into the ingress ports and to clock data out of theegress ports. To use the same clock rate to clock the data out of theegress ports, e.g., to match input and output clock rates, the recoveredclock signals may be multiplexed with the data being connected to theselected egress ports. Thus, every egress port is configured to beclocked by all of the ingress ports, and every ingress clock needs to becompensated for by every egress port. When a large number of ingress andegress ports are being cross connected, collapsing the multipledifferent clock domains is challenging, particularly in an FPGAimplementation with limited clock resources. A 400G cross connect systemwith 10G resolution, for example, involves collapsing 40 different clockdomains.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will be better understood byreading the following detailed description, taken together with thedrawings wherein:

FIG. 1 is a schematic diagram of a cross connect system including atransparent clocking system, consistent with an embodiment of thepresent disclosure.

FIG. 2 is a schematic diagram of a 400G cross connect system with 10Gresolution including transparent clocking, consistent with an embodimentof the present disclosure.

FIG. 3 illustrates one application for a cross connect system,consistent with embodiments of the present disclosure, as an aggregatorand add-drop multiplexer (ADM).

FIG. 4 is an enlarged view of an ODUk cross connect shown in FIG. 3.

FIG. 5 illustrates another application for a cross connect system,consistent with embodiments of the present disclosure, in an opticalservices transport platform (OSTP) for upgrading a reconfigurableoptical add-drop multiplexer (ROADM) system.

DETAILED DESCRIPTION

A cross connect apparatus or system with transparent clocking,consistent with embodiments described herein, connects a selected sourceor ingress port to a selected destination or egress port and clocks dataout of the selected egress port using a synthesized clock that isadjusted to match a recovered clock from the selected ingress port. Atransparent clocking system may generate the synthesized clock signalwith adjustments in response to a parts per million (PPM) rate detectedfor the associated recovered clock signal provided by the selectedingress port. The cross connect system with transparent clocking may bea 400G cross connect system with 10G resolution. The cross connectsystem with transparent clocking may be used in optical transportnetwork (OTN) applications, for example, to provide an aggregator and/oran add-drop multiplexer (ADM) or to provide a reconfigurable opticaladd-drop multiplexer (ROADM) upgrade to a higher data rate (e.g., 10G to100G).

Referring to FIG. 1, a cross connect system 100, consistent withembodiments of the present disclosure, generally includes a plurality ofsource or ingress ports 110-1 to 110-N connected to a plurality ofdestination or egress ports 120-1 to 120-N via a plurality ofmultiplexers 130-1 to 130-N. Any one of the ingress ports 110-1 to 110-Nmay thus be connected to any one of the egress ports 120-1 to 110-N.Thus, the number of potential ingress to egress combinations allows fora large number of potential mappings. For instance, and in onenon-limiting example embodiment, a 40 port system may be capable of 1600potential ingress-egress, mappings (40×40=1600). Other portconfigurations, e.g., 10×10, 20×20, 60×60, are also within the scope ofthis disclosure.

Each of the source/ingress ports 110-1 to 110-N may include circuitryfor receiving data signals and recovering data and clock signals(REC_CLK). Each one of the destination/egress ports 120-1 to 120-N mayinclude circuitry for transmitting data signals that have been clockedusing a synthesized clock signal (TXREF) adjusted to match a recoveredclock signal (REC_CLK) from a selected one of the ingress ports 110-1 to110-N being connected. The cross connect system 100 includes atransparent clocking system 140 for generating the synthesized clocksignals (TXREF) in response to the recovered clock signals (REC_CLK), aswill be described in greater detail below. As generally referred toherein, a transparent clock refers to an approach whereby the TX outputclock (TXREF) for an egress port operates without direct synchronizationwith an associated input clock (REC_CLK) of a mapped ingress port.Instead, the TX output clock (TXREF) may be synthetically generatedbased on a measured clock rate difference (e.g., in parts per million(PPM)) between a recovered clock associated with an ingress port and aTX reference clock, with the TX reference clock have a rate greater thanthe associated input clock. The data being connected to a selectedegress port from a selected ingress port may thus be clockedtransparently through the cross connect system using a consistent andhighly-accurate clock rate without having to multiplex the recoveredclock signals, e.g., without having to maintain a separate clock orotherwise allocate dedicated clock resources for each ingress-egressport combination. Such a system is thus capable of handling multipleclock domains, e.g., up to 40 input/output ports or more, with limitedclock resources, e.g., in a FPGA, Silicon Integrated Circuit (SIC) orother chip implementation having constrained clock resources.Accordingly, N number of ingress ports may be cross connected to Nnumber of egress ports in a 1:1 fashion, with updates to the mappingsbetween input and output ports being dynamic, e.g., based on user input,a remote command, dip switches, and other suitable programmingapproaches. This may allow for ports to be initially cross-coupled in adesired configuration, e.g., during factory configuration or siteinstallation, and optionally reconfigured during operation for purposesof load balancing, traffic rerouting (e.g., in the event of a fault),network topology changes, unit swap-outs, and so on.

Referring to FIG. 2, an embodiment of a cross connect system 200 withtransparent clocking is shown and described in greater detail. As shown,the cross connect system 200 includes a plurality of source/ingressports 210-1 to 210-N coupled to a destination port 220 via a multiplexer230. Although a single multiplexer 230 and destination port 220 isshown, the cross connect system 200 may include a plurality ofmultiplexers 230 connected to a plurality of respective destinationports 220 depending on a desired configuration. Other switching logicmay also be used and the particular embodiment shown should not beconstrued as limiting. The cross connect system 200 may be implemented,whole or in part, within a single package 250. The single package 250may comprise an FPGA, a SIC, or other suitable chip, for example. Insome cases, the reference LO 241 and TX clock generator 246 may beimplemented within separate chips/circuitry.

Each multiplexer 230 multiplexes a plurality of data paths 216-1 to216-N from the plurality of respective source/ingress ports 210-1 to210-N onto a single data path 226 to the respective destination/egressport 220. This defines a static connection that establishes theconnection from ingress port to egress port on the cross connect system200. Each of the data paths 216-1 to 216-N may include a multiple bitbus for connecting to multiple respective destinations.

In the illustrated embodiment showing a 400G cross connect system 200with 10G resolution, 40 source/ingress ports 210-1 to 210-N each receivedata signals at a 10G data rate. Each destination/egress port 220 maythus get its inputs from 40 source/ingress ports 210-1 to 210-N. In thisexample, the source/ingress ports 210-1 to 210-N are connected to 40 bitbus data paths 216-1 to 216-N, respectively. The source/ingress ports210-1 to 210-N may be implemented in client interfaces receiving OTU2data signals as defined by the Optical Transport Network (OTN) standard(also known as ITU-T Recommendation G.709), although other embodimentsare also within the scope of this disclosure.

Each of the source/ingress ports 210-1 to 210-N includes a receiver212-1 to 212-N coupled to an ingress FIFO module 214-1 to 214-N. Thereceivers 212-1 to 212-N receive data signals (e.g., Client0 RX toClient39 RX) and recover the data and clock from the respective receiveddata signals. The recovered data is clocked into the ingress FIFO module214-1 to 214-N of each respective source/ingress port using therecovered clock signal. The data paths 216-1 to 216-N from the ingressFIFO modules 214-1 to 214-N may be multiplexed into the single data path226 to the destination/egress port 220 through a user selectabledestination register. The selection register may include a twodimensional data structure representing all possible ingress-egresscombinations. For each egress port, for example, there may be a 6 bitvector that represents the ID of the ingress port that is feeding it,although other address/register schemes may be used. In the exampleembodiment, there may be 40 selectable destination registers. Eachdestination/egress port 220 includes a transmitter 222 coupled to anegress FIFO module 224. The multiplexed data is clocked into the egressFIFO module 224 from the multiplexed data path 226 and clocked out ofthe egress FIFO module 224 for transmission by the transmitter 222, aswill be described in greater detail below. The receivers andtransmitters may be part of a gigabit transceiver block (GXB).

To provide the transparent clocking, this embodiment of the crossconnect system 200 further includes a local oscillator (LO) 241 orreference LO 241, a parts per million (PPM) detectors module 242, gappedclock enable logic 244, a TX clock generator 246, and control logic 248such as a processor or a dedicated finite state machine (FSM). The TXclock generator 246 may be implemented as a phase locked loop (PLL) orany other circuitry/chip capable of fine-grain PPM adjustment to matchingress-egress clock rates. Although one TX clock generator 246 isshown, this disclosure is not necessarily limited in this regard. Forexample, each TX clock generator 246 may service one or more egressports, and thus, the cross connect system 200 may include N number of TXclock generators. The reference LO 241 provides a local clock signal,running faster than any of the recovered clock signals, to each of theingress FIFO modules 214-1 to 214-N, to each egress FIFO module 224, tothe PPM detectors module 242, and to the TX clock generator 246. Asingle reference LO, e.g., reference LO 241, may be utilized toaccommodate N number of input-output/ingress-egress ports, although insome implementations two or more reference LO ports may be utilizeddepending on a desired configuration. This advantageously avoids thenecessity of having a separate clock maintained for each potentialingress-egress port mapping.

Data may be clocked from a FIFO modules 214-1 to 214-N to a mappedegress queue, e.g., egress FIFO module 224, based on the clock rate ofthe reference LO 241. The PPM detectors module 242 may include a PPMdetector for each of the source/ingress ports 210-1 to 210-N to detect aPPM rate of the recovered clock for each of the source/ingress ports210-1 to 210-N relative to the reference LO 241. The gapped clock enablelogic 244 monitors the FIFO fill levels of the egress FIFO module 224and adjusts the write clock of the egress FIFO module 224 to skip aclock cycle as needed to match the data rate of the selectedsource/ingress port. By way of example, consider a recovered clockequals 40 Gps and the reference LO 241 is operating at a faster ratesuch as 10× the recovered clock. In this example, 1/10^(th) of theoverall clocks cycles may be “skipped” to cause data to be output by atransmitter, e.g., 222, at a matching rate of 40 Gbps. In some cases,the gapped clock enable logic 244 outputs a signal, e.g., a skip signal,to cause one or more clock ticks to be skipped/ignored.

To achieve this matched rate between input and mapped output, the TXclock generator 246 generates the synthesized clock signal (TXREF) forclocking data from the read side of the egress FIFO module 224. Thecontrol logic 248 controls the selection of the source and destinationports, receives the PPM rates from the PPM detectors module 242, andcommunicates with the TX clock generator 246, for example, using I²Ccontroller integration. The control logic 248 may thus select one of thesource ports 210-1 to 210-N via the multiplexer 230 to establish aconnection to the destination port 220, e.g., based on a user-definedmapping, and then pass the detected PPM rate for the selected sourceport to the TX clock generator 246. As discussed above, the PPM rate isrelative to difference between the reference LO 241 and a recoveredclock. The TX clock generator 246 may then adjust or otherwise fine-tunethe synthesized clock (TXREF) based on the PPM difference relative tothe local oscillator signal used to clock data into the egress FIFOmodule 224. The synthesized clock signal (TXREF) thus matches therecovered clock frequency for the selected ingress port being connectedto the egress port, and each of the ingress ports may have its ownsynthesized clock signal generated by the TX clock generator 246 andused as a transmit reference at the connected egress port. The clockingthroughout the cross connect system 200 may thus be transparent suchthat the following equation is satisfied: the recovered clock=the gappedlocal oscillator=TXREF.

FIGS. 3-5 illustrate OTN applications for embodiments of a cross connectapparatus or system with transparent clocking. In these embodiments, thecross connect system apparatus or system is an electrical-switchingbased optical cross connect (OXC).

Referring to FIGS. 3 and 4, a cross connect apparatus or system withtransparent clocking, consistent with embodiments described herein, maybe used as an aggregator and/or ADM. In the illustrated embodiment, asystem 302 includes at least one ODUk cross connect apparatus 300including a plurality of express ports 310 a including pairs of ingressports and egress ports and a plurality of add-drop ports 310 b includingpairs of ingress ports and egress ports. One or more muxponders (MXPs)350 are connected to one or more groups of the express ports 310 a. Oneor more transponders, such as transponder 360 and multi-mode transponder370, are connected to one or more add-drop ports 310 b. In theillustrated embodiment, the ODUk cross connect apparatus 300 is a 400Gcross connect providing 20 OTU2×20 OTU2 connectivity; however, otherembodiments are within the scope of the present disclosure.

Referring to FIG. 5, a cross connect system with transparent clocking,consistent with embodiments described herein, may be used to provide aROADM system upgrade to a higher data rate (e.g., from 10G to 100G). Inthe illustrated embodiment, an optical network 401 includes ROADMs 460coupled to routers 470. To upgrade to 100G, an optical servicestransport platform (OSTP) 402 with an optical cross connect 400 iscoupled between the ROADM 460 and the router 470.

Accordingly, a cross connect apparatus or system with transparentclocking is capable of handling multiple clock domains withoutmultiplexing the recovered clock signals and with limited clockresources (e.g., in a FPGA implementation). This advantageouslyminimizes the overall number of components to accomplish flexible crossconnecting of ports, which reduces the potential for component failureand reduces the overall physical footprint of cross connect circuitry toachieve high-density implementations. An electrical-switching-basedoptical cross connect (OXC) may be useful, for example, in high datarate OTN applications.

In accordance with an aspect of the disclosure an apparatus isdisclosed. The apparatus including a plurality of source ports forreceiving data signals from sources and recovering clock signals fromthe data signals, a plurality of destination ports for transmitting datasignals to destinations, a plurality of multiplexers coupled between thesource ports and the destination ports, the multiplexers beingconfigured to selectively pass the data signals from a selected one ofthe source ports to a selected one of the destination ports, and atransparent clocking system configured to generate synthesized clocksignals adjusted to match recovered clock signals for selected ones ofthe source ports and configured to clock data from selected ones of thedestination ports without multiplexing the recovered clock signals.

In accordance with another aspect of the present disclosure an apparatusis disclosed. The apparatus comprising a plurality of source ports forreceiving data signals and recovering clock signals, each of the sourceports being configured to clock recovered data into an ingress FIFOmodule, at least one destination port for transmitting data signals, theat least one destination port being configured to clock data out of anegress FIFO module, at least one multiplexer coupled between the sourceports and the at least one destination port, the at least onemultiplexer being configured to multiplex a plurality of data paths fromthe plurality of source ports to a single data path to the at least onedestination port and to select one of the source ports for connection tothe destination port, a local oscillator running faster than therecovered clock signals, for clocking data into the egress FIFO module,a parts per million (PPM) detectors module configured to detect PPMrates of selected recovered clock signals, gapped clock enable logicconfigured to adjust a write clock of the egress FIFO module in responseto a data rate of a corresponding selected recovered clock signal, aclock generator configured to generate a synthesized clock signal forthe at least one destination port in response to the detected PPM rateof the selected recovered clock signal, and control logic implemented asa processor or finite state machine, the control logic coupled to thePPM detectors module, the multiplexer, and the clock generator, thecontrol logic being configured to control selection of the source portsvia the multiplexer and to receive the PPM rates and pass the PPM rateof the corresponding selected recovered clock signal to the clockgenerator.

In accordance with another aspect of the present disclosure a system isdisclosed. The system comprising at least one ODUk cross connectapparatus comprising, a plurality of OTU express ports including pairsof ingress ports and egress ports, a plurality of OTU add-drop portsincluding pairs of ingress ports and egress ports, a plurality ofmultiplexers between ingress ports and egress ports, the multiplexersbeing configured to selectively pass data signals from any one of theingress ports to any one of the egress ports, and a transparent clockingsystem configured to generate synthesized clock signals adjusted tomatch recovered clock signals for selected ones of the ingress ports andconfigured to clock data from selected ones of the egress ports withoutmultiplexing the recovered clock signals, at least a first muxpondercoupled to at least a first group of the express ports, and at least onetransponder coupled to at least one of the add-drop ports.

In accordance with another aspect of the present disclosure a system isdisclosed. The system comprising an optical services transport platform(OSTP) configured to be coupled to a reconfigurable optical add-dropmultiplexer (ROADM) and configured to be coupled to a router, and anoptical cross connect apparatus comprising a plurality of ingress portsand a plurality of egress ports, a plurality of multiplexers betweeningress ports and egress ports of the client interfaces, themultiplexers being configured to selectively pass data signals from anyone of the ingress ports to any one of the egress ports, and atransparent clocking system configured to generate synthesized clocksignals adjusted to match recovered clock signals for selected ones ofthe ingress ports and configured to clock data from selected ones of theegress ports without multiplexing the recovered clock signals.

While the principles of the disclosure have been described herein, it isto be understood by those skilled in the art that this description ismade only by way of example and not as a limitation as to the scope ofthe disclosure. Other embodiments are contemplated within the scope ofthe present disclosure in addition to the exemplary embodiments shownand described herein. Modifications and substitutions by one of ordinaryskill in the art are considered to be within the scope of the presentdisclosure, which is not to be limited except by the following claims.

What is claimed is:
 1. A method for providing transparent clockingbetween a plurality of ingress ports and a plurality of egress ports ina cross-connect system, the method comprising: receiving a data signalat a first ingress port of the plurality of ingress ports and recoveringa clock signal from the data signal; generating a synthesized clocksignal based on the recovered clock signal, the synthesized clock signalbeing adjusted to match the recovered clock signal; passing datarepresenting the received data signal from the first ingress port to aselected one of the egress ports; and wherein passing the datarepresenting the received data signal includes clocking the data at theselected one of the egress ports in order to transmit a signal based onthe synthesized clock signal such that the clock frequency of the signaltransmitted by the selected one of the egress ports substantiallymatches the clock frequency of the data signal received at the firstingress port.
 2. The method of claim 1, wherein generating thesynthesized clock signal further comprises determining a differencebetween a measured clock rate of the recovered clock signal and atransmit reference clock associated with the plurality of egress ports,the transmit reference clock having a clock rate greater than themeasured clock rate of the recovered clock signal.
 3. The method ofclaim 2, wherein determining the difference between the measured clockrate of the recovered clock signal and the transmit reference clockfurther comprises comparing a detected parts per million (PPM) rate ofthe recovered clock signal relative to a PPM rate of the transmitreference clock.
 4. The method of claim 2, wherein generating thesynthesized clock signal includes using the difference between themeasured clock rate of the recovered clock signal and the transmitreference clock to determine a number of cycles to skip, and wherein thesynthesized clock signal is introduced by clocking using the transmitreference clock and skipping the determined number of cycles whileclocking with the transmit reference clock such that the clock rate ofthe synthesized clock signal is less than the clock rate of the transmitreference clock and is substantially equal to the clock rate of therecovered clock signal.
 5. The method of claim 1, the method furthercomprising establishing a map in a memory that associates each ingressport of the plurality of ingress ports to one or more egress ports, andwherein selection of the selected one of the ingress ports and theselected one of the egress ports is based on the map.
 6. The method ofclaim 5, further comprising updating the map based on user input todynamically map at least one ingress port of the plurality of ingressports to one or more egress ports of the plurality of egress ports. 7.The method of claim 1, wherein generating a synthesized clock signalfurther comprises generating a synthesized clock signal for each ingressport of the plurality of ingress ports based on an associated recoveredclock signal.
 8. The method of claim 1, wherein passing the datarepresenting the received data signal further includes multiplexing datareceived at each ingress port by a common multiplexer that operates as asingle data path for the plurality of ingress ports.
 9. The method ofclaim 1, wherein generating the synthesized clock signal based on therecovered clock signal further comprises adjusting a clock rate of alocal oscillator associated with the selected one of the egress portssuch that received data gets clocked into an egress queue associatedwith the selected one of the egress ports without multiplexing therecovered clock signal.
 10. A method for providing transparent clockingbetween a plurality of ingress ports and a plurality of egress ports ina cross-connect system, the method comprising: clocking data received ateach ingress port of the plurality of ingress ports into a correspondingingress queue; recovering a clock signal for the data received at eachingress port; and clocking data from each ingress port into acorresponding egress port via a multiplexer and local oscillator, themultiplexer being configured to multiplex data from each ingress queueinto a single data path and the local oscillator used to syntheticallygenerate a transmit clock for each egress port, the syntheticallygenerated transmit clock for each egress port having a clock rate thatsubstantially matches the clock rate of the recovered clock signal foran associated ingress port.
 11. The method of claim 10, whereingenerating the synthetic transmit clock further comprises determining adifference between a measured clock rate of the recovered clock signaland the clock rate of the local oscillator.
 12. The method of claim 11,wherein determining the difference between the measured clock rate ofthe recovered clock signal and the local oscillator further comprisescomparing a detected parts per million (PPM) rate of the recovered clocksignal relative to a PPM rate of the local oscillator.
 13. The method ofclaim 10, the method further comprising establishing a map in a memorythat dynamically associates each ingress port of the plurality ofingress ports to one or more egress ports, and wherein clocking datafrom each ingress port into the corresponding egress port is based onthe map.
 14. The method of claim 13, further comprising updating the mapbased on user input to dynamically adjust associations between eachingress port and corresponding egress ports.
 15. The method of claim 10,wherein synthetically generating the transmit clock for each egress portfurther comprises skipping one or more clock cycles of the localoscillator such that data gets written to each egress port at a clockrate that substantially matches the clock rate of the associatedrecovered clock signal.